Step 1: Add TF2 model
Step 2: Right click on the model in Animator Set Editor and press "Add Team Fortress Item"Step 3: Now you have a menu with lots of options, but for now, choose "Hats" filter
or "Other Cosmetics"
Step 3.1: If you dont want to choose random hat, just search it in "Name Filter"
Step 4: Boom! Now you have a hat on your model!

So, let's say I want to add a Last Breath to this Pyro using this method
Okay aaand.. OH NO ITS BROKEN
Don't worry, just do this before adding the hat:
Right click on the model, Set Body Groups, head, choose <none>
And now it's working!

This trick you'll need to use when hat replaces certain parts of a body and you need to remove those bodyparts using the "Set Body Groups" menu

Congrats! You have learned the first way to add hats!

Let's say you read this guide and updated your SFM content manually and now you want to add the hats from the newer version of Team Fortress 2 to your project.

Now I'll try to add the Field Practice hat for this Medic, using our first way, but oh god!, there is no hat!

Don't Panic! First off in this guide you'll find all the names for TF2 items including newer hats like Field Practice.

Okay so the name for field practice is Spr17_Field_Practice, pretty straight forward one.
Now let's find it in model files...
Bingo! It's right here!
After adding the hat, you should have an image like this:
So now, go the "rootTransform" bone in the character model...
...and on the Timeline click Right Mouse Button and then "Copy Samples"
(using this mode with a green background)
Then go to the hat's bones and also choose "rootTransform"

on the same "Timeline" green background mode, Right Click and then "Delete Samples"
and now "Paste Samples"

Taa-dah! You should now have perfectly placed hat

One last thing to do, is to pull character's bip_head bone to hat's bip_head bone, like this:

And it should then look like this:

You need to do this so you can freely move the HEAD around and have the HAT to perfectly follow it!

Now you know how to add hats with 2 ways that will serve you a very long time

Here are final results:

First way, German Gonzila:


Second way, Field Practice: